The fatigue performance of welded components has been established through the testing of many components in the laboratory. The results have been processed and presented as S-N diagrams for the design and assessment of welded joints (BS 7608:1993). This work was mostly done before the widespread use of computer based stress analysis, so the S-N curves were originally presented in terms of nominal stresses calculated using load over area or basic beam bending theory. Engineers now routinely use Finite Element Analysis (FEA) to analyse structures. The predicted stress distributions are complex so it is often difficult to decide how to extract a suitable stress for use with the published S-N curves.
There is generally a stress concentration at a weld toe, but this distribution can be characterised in terms of the linear equivalent of the real stress distribution at possible sites of fatigue cracking. The value from the linear distribution at the weld toe is called the Structural Stress. Fatigue will initiate at the locations where the structural stress is highest. The structural stress at this location is called the Hot Spot Stress. Worldwide research has developed an understanding of S-N curves plotted using the Hot Spot Stress. It appears that many components have the same fatigue strength as a butt weld when the strengths are expressed in this way.
Extraction of an accurate Hot Spot Stress from an FE model is a critical part of the weld fatigue assessment so it is important that guidelines are provided which ensure that this is done correctly. Engineers face competing demands because a fine FE mesh is needed to make accurate predictions, but, a coarse mesh will be easier to generate and quicker to run. Similarly, the calculation of Hot Spot Stresses on many practical structures is a time consuming process, because of the large number of welds and the significant number of load cases experienced in service.
It is therefore important that methods of reviewing FEA results for weld fatigue assessments should be efficient, robust and clear so that design engineers can reliably determine the performance of structures.
Three methods of calculating the Hot Spot Stress have been developed:
- Surface Stress Extrapolation uses the predicted stress at two locations in an extrapolation to obtain a stress at the weld toe
- Through Thickness Integration uses the predicted stress distribution in the plate below the weld toe. The stresses are integrated to obtain a force and a moment, and these are used to obtain the structural stress at the weld toe
- The Nodal Force technique is similar because a force and a moment are calculated for a section at the weld toe. However, this method is claimed to be more accurate
FEA has provided a means for the accurate assessment of weld fatigue for many structures, but there is still a challenge in some industries where the structures are large and contain many welds. Further work is needed to ensure that suitable meshes are used for these structures and to reduce the effort needed to review the complete set of results.
Fatigue of fillet welds across the throat from the weld root requires further research.
BS 7608 was developed by TWI based mainly upon tests conducted in the TWI laboratories. TWI was the first organisation to interpret S-N curves using the Paris law. This technique has now been adopted by other organisations and is now gaining interest as a potential method of developing S-N curves for new geometries.
Universal S-N curves provide industry with a basis for the design of welded structures with reduced requirement for time consuming fatigue tests.
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